The invention relates to a method for detecting defective injectors for feeding fuel into the combustion chambers of an internal combustion engine, in particular in a motor vehicle, and to a corresponding engine test device for detecting defective injectors.
In the most contemporary diagnostic methods it is not possible to identify defective injectors in internal combustion engines without the injectors having to be removed from the engine for this purpose. Accordingly, in the case of malfunctions of an internal combustion engine which could be caused by defective injectors, these nozzles often have to be replaced based on suspicion of a fault. In many cases this leads to a situation in which an incorrect replacement of injectors occurs when there is no defect in the injectors. As a consequence, repeated repairs are necessary. In addition, the warranty costs of the engine manufacturer are increased leading to unnecessary repairs.
The object of the invention is therefore to provide a method and a corresponding engine test device with which defective injectors can be detected easily and reliably without removing the injectors from the internal combustion engine.
This and other objects are achieved by a method and an engine test device in accordance with embodiments of the invention.
Within the scope of executing the method according to the invention, the internal combustion engine is operated in the idling mode at an idling speed which is increased compared to the normal idling mode. This speed is preferably 1000 revolutions per minute or higher, in particular 1150 revolutions per minute or higher. In this way, stable operation of the internal combustion engine is ensured when the method is carried out. When the method according to the invention is carried out the injection pressure of the injectors is also preferably adjusted to a predetermined value (in particular 150 bar). In addition, during the method the mixture control of the internal combustion engine is deactivated. The mixture composed of fuel and air in the respective combustion chambers is adjusted by this control in the normal operating mode of the internal combustion engine. The adjustment of the mixture when the mixture control is activated takes place here on the basis of a characteristic value which depends on the ratio of air to fuel in the mixture and preferably increases when there is a relatively large portion of air in the mixture. The characteristic value is measured jointly for one or more injection groups composed of a plurality of injectors of the internal combustion engine, in each case for all the injectors of the corresponding injection group. By means of the joint measurement of the characteristic value for all the injectors of a respective injection group, an average characteristic value is detected for the mixture during the combustion in the combustion chambers into which the injectors of the corresponding injection group inject. The characteristic value is preferably the lambda value which is known per se and which represents the ratio of air to fuel of the mixture in comparison with a stoichiometric mixture, and to which there is usually recourse to the mixture control. The lambda value is generally determined by use of a lambda probe which is arranged, for example, in a common exhaust gas line of the combustion chambers, into which exhaust gas line the injectors of the corresponding injection group inject. The lambda probe measures the residual oxygen content in the exhaust gas in the exhaust gas line.
Within the scope of the method according to the invention, the steps a) to c) explained below are carried out in each case for at least some of the injection groups and, in particular, for all the injection groups of the internal combustion engine when the measurement of the characteristic value is activated. Although these steps are carried out when the mixture control is deactivated, the measurement of the characteristic value continues to take place. In addition it is to be noted that the designation of the steps by the letters a) to c) serves merely for better referencing of corresponding features of the steps, without necessarily a time sequence for carrying out the steps being defined as a result.
In step a) of the method, each injector of the respective injection group is switched off selectively, which indicates here and below that at a respective time or in a respective time interval only the injection of a single injector is switched off. In this case, the change in the characteristic value is detected when the respective injector is switched off. The injectors of a respective injection group are preferably switched off directly one after the other, in order to avoid fluctuations during the measurement of the characteristic value. In order to detect the change in the characteristic value, in particular the characteristic value is measured when the respective injector is switched off, and the characteristic value is measured before the respective injector is switched off (i.e. when all the injectors of the respective injection group are switched on). The change in the characteristic value represents the difference (for example the difference in absolute terms) between these characteristic values.
In step b) of the method according to the invention, a fault criterion is checked. The fault criterion is satisfied when the change in the characteristic value for the respective switched-off injector exceeds or undershoots a predetermined amount. The fault criterion is preferably checked to determine whether the predetermined amount is exceeded. In this case, the particularly relevant fault cases of injectors which inject too much fuel into the corresponding combustion chamber compared to their technical specification can be detected. Nevertheless, under certain circumstances defective injectors which inject too little fuel into the combustion chamber compared to their technical specification can also be detected with a fault criterion based on the undershooting of a predetermined amount. In step c) of the method, in the event of the fault criterion being satisfied, a defect of the respective (switched-off) injector is finally detected.
The method according to the invention is based on the realization that a relatively large or relatively small change in a characteristic value which is dependent on the mixture ratio when a respective injector is switched off makes it possible to infer abnormal operation of the injector in the corresponding injection group, with the result that in this way a defect in the switched-off injector can be detected.
The method according to the invention is preferably used for the internal combustion engine of a motor vehicle, but can, under certain circumstances, also be used for other internal combustion engines, such as, for example, ship's engines, power station engines, test bench engines and the like. With the method according to the invention it is possible to detect a wide variety of defects of injectors. In particular, the method is suitable for injectors in the form of high-pressure injectors, preferably piezo injectors, in a directly injecting internal combustion engine and, in particular, in a spark ignition engine. Basically, with the method according to the invention it is possible to detect all defects of injectors which give rise to a quantity deviation of the supplied fuel such as, for example, leaks in the injectors. In addition, the method can also be used to detect defects in injectors which do not inject directly into the combustion chamber.
In one particularly preferred embodiment, the internal combustion engine is operated before the selective switching off of the injectors until heating of one or more catalytic converters for purifying the exhaust gas of the internal combustion engine is ended. This ensures correct measurement of the characteristic values.
In a further preferred embodiment, the internal combustion engine is operated with execution of the method in the idling mode by connecting an increased load, wherein the load is connected, in particular, by activation of one or more electrical consumers of a motor vehicle which are driven by the internal combustion engine. By connecting the load, a stable speed of the engine with small fluctuations is achieved.
The fault criterion which is checked in step b) can be configured in different ways. Under certain circumstances, the predetermined amount can be a predefined value of the change in the characteristic value, the exceeding or undershooting of which satisfies the fault criterion. However, the predetermined amount can preferably be a relative amount including the characteristic values when the other injectors of the injection groups are switched off. In other words, in this case the fault criterion is satisfied in step b) when the change in the characteristic value for the respective switched-off injector relative to the change in the characteristic value when the other injectors of the respective injection group are switched off exceeds or undershoots a predetermined amount. By means of this relative amount it is possible to detect very efficiently abnormal behavior of an injector in comparison with the other injectors of the injection group, and therefore to detect a defect.
In a further refinement of the method according to the invention, the predetermined amount is given by a limiting value above 100%, wherein the fault criterion is satisfied when the percentage value of the change in the characteristic value for the respective switched-off injector with respect to the smallest change which has occurred in the characteristic value exceeds the limiting value when the respective injectors of the respective injection group are switched off. This makes it possible to detect efficiently defective injectors which are operating in a rich mode and which are injecting too much fuel compared to their specification.
In order to detect defective injectors which are operating in a lean fashion and which inject too little fuel compared to their specification, the predetermined amount is preferably given by a limiting value below 100%, wherein the fault criterion is satisfied when the percentage value of the change in the characteristic value for the respective switched-off injector with respect to the largest change which has occurred in the characteristic value undershoots the limiting value when the respective injectors of the respective injection group are switched off.
In a further variant of the method according to the invention, the predetermined amount is given by a threshold value, wherein the fault criterion is satisfied when the change in the characteristic value for the respective switched-off injector minus the changes in the characteristic value when the other injectors of the respective injection group are switched off exceeds the threshold value (in the case of defective injectors which are operating in an enriched mode) or undershoots said threshold value (in the case of defective injectors which are operating in a lean mode).
In a further preferred variant, the limiting value described above or the threshold value described above is dependent on a measured characteristic value when all the injectors of the respective injection group are switched on. This measured characteristic value is generally determined directly before the selective switching off of the injectors and used to determine the change in the characteristic value. The limiting value or the threshold value preferably increases monotonously when the measured characteristic values rise, and in the process said limiting value or threshold value also increases at least in a sub-interval of the measured characteristic values. Monotonously rising means here that the limiting value or threshold value can also remain constant in certain intervals of the characteristic values but never decreases. This variant is used, in particular, for detecting defective injectors which operate in a rich mode insofar as the characteristic value increases when there is a relatively large portion of air in the mixture. In this context, it is taken into account that in the case of relatively high characteristic values and therefore relatively lean mixtures in the normal operating mode of the internal combustion engine, enrichment (increasing the fed-in mass of fuel) of the mixture is carried out by means of the mixture control, with the result that there is a lower probability of ignition misfires and therefore of faulty operation of the engine. Under certain circumstances, in the case of rising measured characteristic values the limiting value or threshold value can also drop monotonously and in the process also decrease at least in a sub-interval of the measured characteristic values. Monotonously dropping means in this context that the limiting value or threshold value can also, under certain circumstances, be constant in certain intervals of the characteristic values, but never increases. This variant is used, in particular, for the detection of defective injectors which are operating in a lean mode, insofar as the characteristic value increases when there is a relatively large portion of air in the mixture.
In one particularly preferred variant, the method according to the invention is carried out in the cold operating mode of the injectors. In this context, such defects of injectors which occur only in the cold operating mode, such as, for example, temperature-dependent deviation of the quantity of fuel fed in via the injectors, can be detected efficiently. In order to ensure that the method also actually operates in the cold operating mode, in one preferred variant the result of the method or partial results of the method is/are categorized as invalid if a cold operating mode is not continuously present during the execution of the method. This ensures that defects which occur only in the cold operating mode are reliably detected. A cold operating mode of the injectors is preferably detected when an injector temperature is below a predetermined threshold, in particular below 50° C. The injector temperature is preferably estimated here on the basis of the temperature of the internal combustion engine at the start of the method and the time period up to the ending of the method. In particular, this estimate can also be based here on characteristic curves with a constant injector temperature, which curves are represented in a diagram as a function of the temperature of the internal combustion engine at the start of the method and the time period up to the ending of the method. On the basis of a corresponding internal combustion engine temperature and a time period, a point in the diagram is obtained which lies on one of the characteristic curves and therefore specifies the injector temperature.
The method according to the invention can also be carried out in the hot operating mode of the injectors. A hot operating mode is detected here, in particular, when no cold operating mode is detected, wherein the cold operating mode can be detected on the basis of the variant described above. Alternatively or additionally, a hot operating mode can be detected when the temperature of the coolant of the internal combustion engine exceeds a predetermined value (for example 90° C.) and/or the travel time of a motor vehicle in which the internal combustion engine is installed exceeds a predetermined time period (for example 30 min) directly before the start of the method.
The method according to the invention can, for example, firstly be carried out in the hot operating mode of the injectors, wherein in the cold operating mode the steps a) to c) of the method are subsequently carried out only for injectors for which a defect has not been detected in the hot operating mode.
The method according to the invention operates, for example, on a vehicle-external engine test device which is used, for example, in a vehicle repair workshop. The engine test device is connected in a wireless or wired fashion to the engine controller of the vehicle via a corresponding vehicle interface and can by this means control the operation of the vehicle and call measured values from the engine controller. However, it would also be conceivable for the method to operate on the engine control unit of the vehicle.
In addition to the method described above, the invention also relates to an engine test device for detecting defective injectors for feeding fuel into the combustion chambers of an internal combustion engine. The engine test device is configured to carry out the method according to the invention or one or more preferred variants of the method according to the invention. The engine test device can, for example, be the external engine test device which has already been described above and can, if appropriate, also be integrated into the vehicle.
The invention also relates to a motor vehicle having an internal combustion engine and injectors for feeding fuel into the combustion chambers of the internal combustion engine, wherein the motor vehicle comprises the engine test device described above.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.